As a hydraulic drive system for a construction machine such as hydraulic excavator, one having a regulator that controls the capacity (flow rate) of a hydraulic pump in such a manner that the delivery pressure of the hydraulic pump becomes higher than a maximum load pressure of a plurality of actuators by a target differential pressure is widely used, and this is called load sensing control. Patent Document 1 describes a two-pump load sensing system in a hydraulic drive system for a construction machine provided with a regulator for performing such a load sensing control, in which two hydraulic pumps are provided, and the respective two hydraulic pumps perform the load sensing control.
Besides, in a regulator of a hydraulic drive system for a construction machine, normally, a torque control is conducted such that the absorption torque of a hydraulic pump does not exceed a rated output torque of a prime mover, by decreasing the capacity of the hydraulic pump as the delivery pressure of the hydraulic pump rises, thereby to prevent stoppage of the prime mover (engine stall) due to an overtorque. In the case where the hydraulic drive system is provided with two hydraulic pumps, the regulator of one hydraulic pump performs a torque control (total torque control) by using not only its own delivery pressure but also a parameter concerning the absorption torque of the other hydraulic pump, thereby to attain both prevention of stoppage of the prime mover and effective utilization of a rated output torque of the prime mover.
For instance, in Patent Document 2, a total torque control is carried out by introducing the delivery pressure of one of the two hydraulic pumps to the regulator of the other hydraulic pump through a pressure reduction valve. A set pressure of the pressure reduction valve is fixed, and this set pressure is set at a value simulating a maximum torque in the torque control of the regulator of the other hydraulic pump. This ensures that in an operation of driving only the actuators concerning the one hydraulic pump, the one hydraulic pump can effectively use substantially the whole of the rated output torque of the prime mover, and, in a combined operation of simultaneously driving the actuators concerning the other hydraulic pump, the absorption torque of the whole of the pumps does not exceed the rated output torque of the prime mover, so that stoppage of the prime mover can be prevented from occurring.
In Patent Document 3, in order to carry out a total torque control for two variable displacement hydraulic pumps, the tilting angle of the other hydraulic pump is detected as an output pressure of a pressure reduction valve, and the output pressure is introduced to the regulator of the one hydraulic pump. In Patent Document 4, control accuracy of a total torque control is enhanced by detecting the tilting angle of the other hydraulic pump by replacing the tilting angle with the arm length of an oscillating arm.